[SPARK-5279][SQL] Use java.math.BigDecimal as the exposed Decimal type.

[rxin]

No description provided.

[SparkQA]

Test build #25713 has started for PR 4092 at commit 10cb496.

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[SparkQA]

Test build #25713 has finished for PR 4092 at commit 10cb496.

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[SparkQA]

Test build #25720 has started for PR 4092 at commit 27b08c9.

• This patch merges cleanly.

[SparkQA]

Test build #25720 has finished for PR 4092 at commit 27b08c9.

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• This patch merges cleanly.
• This patch adds no public classes.

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[scwf]

here we also need update the sql-doc

[rxin]

Yup. I was thinking I'd work on those towards the end. But if you are interested in submitting patches to fix sql doc as we go along, please do so. Thanks.